1//===- llvm/ADT/PostOrderIterator.h - PostOrder iterator --------*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// This file builds on the ADT/GraphTraits.h file to build a generic graph
11// post order iterator.  This should work over any graph type that has a
12// GraphTraits specialization.
13//
14//===----------------------------------------------------------------------===//
15
16#ifndef LLVM_ADT_POSTORDERITERATOR_H
17#define LLVM_ADT_POSTORDERITERATOR_H
18
19#include "llvm/ADT/GraphTraits.h"
20#include "llvm/ADT/iterator_range.h"
21#include "llvm/ADT/Optional.h"
22#include "llvm/ADT/SmallPtrSet.h"
23#include <iterator>
24#include <set>
25#include <utility>
26#include <vector>
27
28namespace llvm {
29
30// The po_iterator_storage template provides access to the set of already
31// visited nodes during the po_iterator's depth-first traversal.
32//
33// The default implementation simply contains a set of visited nodes, while
34// the External=true version uses a reference to an external set.
35//
36// It is possible to prune the depth-first traversal in several ways:
37//
38// - When providing an external set that already contains some graph nodes,
39//   those nodes won't be visited again. This is useful for restarting a
40//   post-order traversal on a graph with nodes that aren't dominated by a
41//   single node.
42//
43// - By providing a custom SetType class, unwanted graph nodes can be excluded
44//   by having the insert() function return false. This could for example
45//   confine a CFG traversal to blocks in a specific loop.
46//
47// - Finally, by specializing the po_iterator_storage template itself, graph
48//   edges can be pruned by returning false in the insertEdge() function. This
49//   could be used to remove loop back-edges from the CFG seen by po_iterator.
50//
51// A specialized po_iterator_storage class can observe both the pre-order and
52// the post-order. The insertEdge() function is called in a pre-order, while
53// the finishPostorder() function is called just before the po_iterator moves
54// on to the next node.
55
56/// Default po_iterator_storage implementation with an internal set object.
57template<class SetType, bool External>
58class po_iterator_storage {
59  SetType Visited;
60
61public:
62  // Return true if edge destination should be visited.
63  template <typename NodeRef>
64  bool insertEdge(Optional<NodeRef> From, NodeRef To) {
65    return Visited.insert(To).second;
66  }
67
68  // Called after all children of BB have been visited.
69  template <typename NodeRef> void finishPostorder(NodeRef BB) {}
70};
71
72/// Specialization of po_iterator_storage that references an external set.
73template<class SetType>
74class po_iterator_storage<SetType, true> {
75  SetType &Visited;
76
77public:
78  po_iterator_storage(SetType &VSet) : Visited(VSet) {}
79  po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {}
80
81  // Return true if edge destination should be visited, called with From = 0 for
82  // the root node.
83  // Graph edges can be pruned by specializing this function.
84  template <class NodeRef> bool insertEdge(Optional<NodeRef> From, NodeRef To) {
85    return Visited.insert(To).second;
86  }
87
88  // Called after all children of BB have been visited.
89  template <class NodeRef> void finishPostorder(NodeRef BB) {}
90};
91
92template <class GraphT,
93          class SetType =
94              SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
95          bool ExtStorage = false, class GT = GraphTraits<GraphT>>
96class po_iterator
97    : public std::iterator<std::forward_iterator_tag, typename GT::NodeRef>,
98      public po_iterator_storage<SetType, ExtStorage> {
99  typedef std::iterator<std::forward_iterator_tag, typename GT::NodeRef> super;
100  typedef typename GT::NodeRef NodeRef;
101  typedef typename GT::ChildIteratorType ChildItTy;
102
103  // VisitStack - Used to maintain the ordering.  Top = current block
104  // First element is basic block pointer, second is the 'next child' to visit
105  std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
106
107  void traverseChild() {
108    while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
109      NodeRef BB = *VisitStack.back().second++;
110      if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
111        // If the block is not visited...
112        VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
113      }
114    }
115  }
116
117  po_iterator(NodeRef BB) {
118    this->insertEdge(Optional<NodeRef>(), BB);
119    VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
120    traverseChild();
121  }
122
123  po_iterator() = default; // End is when stack is empty.
124
125  po_iterator(NodeRef BB, SetType &S)
126      : po_iterator_storage<SetType, ExtStorage>(S) {
127    if (this->insertEdge(Optional<NodeRef>(), BB)) {
128      VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
129      traverseChild();
130    }
131  }
132
133  po_iterator(SetType &S)
134      : po_iterator_storage<SetType, ExtStorage>(S) {
135  } // End is when stack is empty.
136
137public:
138  typedef typename super::pointer pointer;
139
140  // Provide static "constructors"...
141  static po_iterator begin(GraphT G) {
142    return po_iterator(GT::getEntryNode(G));
143  }
144  static po_iterator end(GraphT G) { return po_iterator(); }
145
146  static po_iterator begin(GraphT G, SetType &S) {
147    return po_iterator(GT::getEntryNode(G), S);
148  }
149  static po_iterator end(GraphT G, SetType &S) { return po_iterator(S); }
150
151  bool operator==(const po_iterator &x) const {
152    return VisitStack == x.VisitStack;
153  }
154  bool operator!=(const po_iterator &x) const { return !(*this == x); }
155
156  const NodeRef &operator*() const { return VisitStack.back().first; }
157
158  // This is a nonstandard operator-> that dereferences the pointer an extra
159  // time... so that you can actually call methods ON the BasicBlock, because
160  // the contained type is a pointer.  This allows BBIt->getTerminator() f.e.
161  //
162  NodeRef operator->() const { return **this; }
163
164  po_iterator &operator++() { // Preincrement
165    this->finishPostorder(VisitStack.back().first);
166    VisitStack.pop_back();
167    if (!VisitStack.empty())
168      traverseChild();
169    return *this;
170  }
171
172  po_iterator operator++(int) { // Postincrement
173    po_iterator tmp = *this;
174    ++*this;
175    return tmp;
176  }
177};
178
179// Provide global constructors that automatically figure out correct types...
180//
181template <class T>
182po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }
183template <class T>
184po_iterator<T> po_end  (const T &G) { return po_iterator<T>::end(G); }
185
186template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {
187  return make_range(po_begin(G), po_end(G));
188}
189
190// Provide global definitions of external postorder iterators...
191template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
192struct po_ext_iterator : public po_iterator<T, SetType, true> {
193  po_ext_iterator(const po_iterator<T, SetType, true> &V) :
194  po_iterator<T, SetType, true>(V) {}
195};
196
197template<class T, class SetType>
198po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {
199  return po_ext_iterator<T, SetType>::begin(G, S);
200}
201
202template<class T, class SetType>
203po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {
204  return po_ext_iterator<T, SetType>::end(G, S);
205}
206
207template <class T, class SetType>
208iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) {
209  return make_range(po_ext_begin(G, S), po_ext_end(G, S));
210}
211
212// Provide global definitions of inverse post order iterators...
213template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
214          bool External = false>
215struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
216  ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
217     po_iterator<Inverse<T>, SetType, External> (V) {}
218};
219
220template <class T>
221ipo_iterator<T> ipo_begin(const T &G) {
222  return ipo_iterator<T>::begin(G);
223}
224
225template <class T>
226ipo_iterator<T> ipo_end(const T &G){
227  return ipo_iterator<T>::end(G);
228}
229
230template <class T>
231iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) {
232  return make_range(ipo_begin(G), ipo_end(G));
233}
234
235// Provide global definitions of external inverse postorder iterators...
236template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
237struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
238  ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
239    ipo_iterator<T, SetType, true>(V) {}
240  ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :
241    ipo_iterator<T, SetType, true>(V) {}
242};
243
244template <class T, class SetType>
245ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {
246  return ipo_ext_iterator<T, SetType>::begin(G, S);
247}
248
249template <class T, class SetType>
250ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {
251  return ipo_ext_iterator<T, SetType>::end(G, S);
252}
253
254template <class T, class SetType>
255iterator_range<ipo_ext_iterator<T, SetType>>
256inverse_post_order_ext(const T &G, SetType &S) {
257  return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));
258}
259
260//===--------------------------------------------------------------------===//
261// Reverse Post Order CFG iterator code
262//===--------------------------------------------------------------------===//
263//
264// This is used to visit basic blocks in a method in reverse post order.  This
265// class is awkward to use because I don't know a good incremental algorithm to
266// computer RPO from a graph.  Because of this, the construction of the
267// ReversePostOrderTraversal object is expensive (it must walk the entire graph
268// with a postorder iterator to build the data structures).  The moral of this
269// story is: Don't create more ReversePostOrderTraversal classes than necessary.
270//
271// Because it does the traversal in its constructor, it won't invalidate when
272// BasicBlocks are removed, *but* it may contain erased blocks. Some places
273// rely on this behavior (i.e. GVN).
274//
275// This class should be used like this:
276// {
277//   ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create
278//   for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
279//      ...
280//   }
281//   for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {
282//      ...
283//   }
284// }
285//
286
287template<class GraphT, class GT = GraphTraits<GraphT>>
288class ReversePostOrderTraversal {
289  typedef typename GT::NodeRef NodeRef;
290  std::vector<NodeRef> Blocks; // Block list in normal PO order
291
292  void Initialize(NodeRef BB) {
293    std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
294  }
295
296public:
297  typedef typename std::vector<NodeRef>::reverse_iterator rpo_iterator;
298
299  ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
300
301  // Because we want a reverse post order, use reverse iterators from the vector
302  rpo_iterator begin() { return Blocks.rbegin(); }
303  rpo_iterator end() { return Blocks.rend(); }
304};
305
306} // end namespace llvm
307
308#endif // LLVM_ADT_POSTORDERITERATOR_H
309